FACTBOX-Key facts on immigration in the United States

(Reuters) - The following are some facts on immigration in the United States, where pro-immigrant activists plan nationwide rallies on May 1 to pressure the government to overhaul laws affecting immigrants.

*There were an estimated 34.2 million immigrants in the United States in 2004, according to the U.S. Census Bureau. Of these, 18.3 million came from Latin America, 8.7 million from Asia and 4.7 million from Europe.

*An estimated 11 million to 12 million undocumented immigrants live and work in the United States, or around one in every 20 workers, according to a study by the Pew Hispanic Centre based on government figures.

*Some 1.1 million people were arrested crossing illegally over the porous U.S.-Mexico border last year, most of them from Mexico and Central America.

*In 2006, U.S. President George W. Bush proposed an overhaul of U.S. immigration laws, offering a path to citizenship for many undocumented immigrants, together with a guest worker program, and tougher border and workplace enforcement.

*The legislation was opposed by Republican lawmakers in the House of Representatives who backed a tougher security program but opposed any effort to grant citizenship rights to illegal immigrants.

*A bipartisan bill on immigration is expected to be presented to U.S. lawmakers in May.